The present invention relates to a projection lens system of a projector that enlarges and projects an image displayed on a light valve onto a screen.
Conventionally, a liquid crystal display device is often used as the light modulator (light valve) of a projector. In recent years, in place of a liquid crystal display device, apparatuses equipped with a plurality of elements that uses a micromachining technique and change mechanically the directions of reflection of light for forming images have been realized. One example of such an apparatus is a DMD (digital mirror device, foil deforming device or display) in which minute mirror-surface elements (micromirrors) are arranged in an array corresponding to pixels and an image is displayed by controlling the angles of the respective mirror surfaces. Compared to a liquid crystal display device, a light modulator with micromirrors for making pixels has faster response and produces a brighter image, so that a light modulator of such type is suited to the realization of a compact projector with high luminance and high image quality.
To enable a dichroic prism to favorably split light into colors and to produce a bright image using the display characteristics of a liquid crystal display device, the incident side of a projection lens system is normally designed so as to be telecentric. On the other hand, to suppress the production of ghosts and blurring of colors due to the dichroic film, the use of a non-telecentric projection system has also been proposed.
In a projector where color images are displayed by time division using a DMD as the light valve, by suitably selecting the illumination system and optical system, it is possible to use a non-telecentric optical system as the projection lens system. When a non-telecentric optical system is applied, it is possible to use a lens group with a small diameter on the light valve side of the projection lens system, and therefore the entire lens system can be made compact. For a so-called “retrofocus lens system” composed of two lens groups with respectively negative and positive refractive powers disposed in that order from the screen side of the lens system, it is easy to provide a long back focus on the image-reducing side (i.e., the incident side) and to make the lens system telecentric, and therefore such retrofocus lens systems are often used as projector lenses. Since this type of lens system can easy increase the field angle, a retrofocus lens systems are suited to not only telecentric system but also non-telecentric system. However, in a non-telecentric optical system, image height has a much larger effect than for telecentric optical systems, and therefore it becomes difficult to correct aberration.
In addition, among domestic electrical appliances, there is demand for a compact display system with a large screen where the distance between the screen and projector is short. In particular, there is demand for a compact wide-angle lens system that can be used in a rear projector where the screen is integrally provided. A projector lens that is non-telecentric on the incident side can be produced with a compact overall structure, and by positioning the lens group of the projection side close to the pupil position on the screen side, it is possible to produce a wide angle lens system with a large field angle. However, in addition to the incident side not being telecentric, as the field angle of the lens group on the screen side increases, it becomes increasingly difficult to correct curvature of field in response to changes in image height. To correct curvature of field, it is necessary to reduce the Petzval sum, and to do so, it is necessary to dispose concave surfaces with sufficiently high curvature with respect to the convex surfaces and/or to provide sufficient distance between concave and convex lenses, resulting in an increase in the number of lenses and in an increase in the length of the lens system.
As a compact, wide-angle lens system, it is preferable to use a simple combination of lens groups with respectively negative and positive refractive powers from the screen side. A negative lens on the screen side has the largest diameter out of the lenses in the lens system, with the diameter of such lens further increasing as the field angle is increased. Accordingly, when a surface with large curvature is used, both design and manufacturing become difficult. A plastic lens is preferable for the lens or lenses on the screen side in view of the cost and weight of large-diameter, however, changes in temperature have an especially large effect on plastic lens. Basing on the above conditions, for the first lens group on the screen side, it is difficult to achieve a sufficient refractive power for correcting aberration.
On the other hand, if many convex/concave lenses are used in the second lens group on the light valve side, to provide sufficient distance between the lenses, it becomes similar arrangement of a system that has a multiple-group such as a negative-positive-negative-positive, so that a compact lens system cannot be realized.
In addition, to reduce the Petzval sum to correct the curvature of field, it is preferable to dispose convex and concave lenses a sufficient distance apart, but when many lenses are used, the overall length of the lens system becomes too long.
It is an object of the present invention to provide a compact, wide-angle lens system with a two-lens group construction that is non-telecentric on the incident side. It is a further object of this invention to provide a projector that is more compact and can display sharp images.